Among the many types of the fluid level sensors for automotive applications, a capacitive level sensor is one of the simplest sensors considering its shape and size. Other sensors require a float mechanism, or series of switches, which change states based upon whether the fluid level increases or decreases. Capacitive sensors do not require the float mechanism or the series of small switches, but instead use simple implementation of printed pattern of electrodes on a typical PCB (printed circuit board), so that the design and implementation is simple and compact. Capacitive based fluid sensors are based upon the change in the capacitance within a given dielectric. As fluid levels change along the generally vertical electrodes of capacitive plates of the sensor, the amount of the capacitance increases or decreases in the sensor's capacitors. That amount of capacitance is now proportional to the level of the fluid
Capacitive based fluid sensors have a practical application in the automotive industry such as within engines, transmissions, gear boxes, and fuel tanks. Essentially, anywhere there is a need to determine a fluid level within a given component or a machine. Because of the capacitive properties of these sensors, they provide real time data to a user or a processing chip.
Traditional capacitive based fluid level sensors that are in contact/submerged in oil have several issues despite the simplest design and the applications. First, the traditional capacitive fluid level sensor cannot measure the same level of the fluid when the fluid has been changed from one fluid to another fluid because those fluids can have a different dielectric constant that defines the capacitance so that the capacitance is proportional to the level. Further, as the choice of the fluid is up to the user, the dielectric constant that depends on the user's choice of the fluid. Another issue is that as the temperature of a dielectric changes so does the value of the dielectric constant. Many of the existing traditional designs of the capacitive level sensor cannot account for the value change in the dielectric constant, for temperature swings, or for when the user chooses any type of the engine oils ranging from a typical mineral oil to a synthetic oil, which may differ from two to four in the relative dielectric constant.
Further, other issues for the traditional capacitive based fluid level sensors that is in contact/submerged in oil are that, over time, the value of the dielectric constant and the resistivity of the fluid in general changes due to deterioration as a result from an accumulation of metal debris, black carbon, or acidification of the engine oil due to the intrusion of the chemicals of burned fuel from the engine. Moreover, traditional capacitive based fluid sensors are not able to compensate for foreign contaminants that alter the value of the dielectric constant in other automotive application like a windshield wash fluid level sensing due to harsh environment of the under-the-hood.
Traditional capacitive sensors have a high noise floor, more than any other type of the implementation, because the source impedance of the capacitive sensor is very high so that the EMC (electromagnetic compatibility) design is very difficult. Therefore, the incoming noise cannot be easily drained due to the high source impedance. As such, a high source impedance results in the output of the sensor having a high level of noise, which becomes unstable. For example, the impedance of a single capacitor powered by a DC power source has essentially infinity as its impedance. As a result, the frequency is traditionally driven at frequencies in the range of 10 kHz.
Accordingly, it is desirable to have a sensor that can compensate for these changes in the value of the dielectric constant, have the versatility of using a sensor driven by a high RF frequency and provide an accurate fluid level measurement.